DESCRIPTION: X-linked mental retardation (XLMR) is the most common cause of inherited mental retardation in males and plays a major etiologic role in the known 25-50 percent excess of retarded males over retarded females. One hundred and twenty-seven XLMR disorders are known. Fifty- three disorders or families with nonspecific XLMR have been regionally mapped and 15 XLMR genes have been cloned. The purpose of the present study is to better characterize their clinical and neurobehavioral phenotype, effect correlations between these parameters, map additional XLMR syndromes and nonspecific disorders, and to further narrow their localizations so that it will become possible to clone additional genes. In the first five years of the study, 28 XLMR syndromes and 19 families with nonspecific XLMR have been studied. A total of 62 families have been entered into the study; 34 have an actual or projected Lod score over 2.0. Ten new syndromes or nonspecific XLMR families with gene localizations have been described and seven syndromes have been significantly redefined. A total of ten new localizations have been effected. Better localization has been accomplished in two additional disorders. In the next five years of the study it is planned to include 25 more large families with either specific syndromes or nonspecific XLMR for linkage studies. Smaller families will also be included for linkage studies if they have a specific syndrome, or for exclusion mapping, if classified as nonspecific. At least four genes will be cloned: Allan- Herndon-Dudley syndrome (Xq21); Mohr-Tranebjaerg syndrome (Xq22); Snyder-Robinson syndrome (Xp21.2- p22.2) and a gene for MR localized to Xq12 in a 1000 kb deletion. Over 90 families will be included in the study over the ten year period. The study is a collaborative project between the University of Miami, Greenwood Genetic Center, University of Florida in Gainesville and Case Western Reserve University, and will be part of an international collaboration for mapping XLMR disorders.